1. Field of the Invention
The present invention involves a programmable modem for digital data, a method of using the modem, and a manner in which to design an appropriate modem. More specifically, the present invention relates to a programmable digital modem using spread spectrum techniques and being specifically programmable to alter the parameters of the modem to improve performance.
2. Description of Related Art
There are a number of systems and methods for communicating information using spread spectrum communication techniques. The direct sequence spread spectrum technique is a digital modulation technique in which a digital signal is spread over a wide frequency band so that it has a noise-like spectrum. This is done by breaking up each data bit into multiple sub-bits (commonly called chips, and referred to in this application as PN code bits or chips (Pseudo Noise code bits)) that are then modulated and up-converted to a carrier frequency. By using orthogonal codes for different communication links, the same frequency band can be used for different simultaneous links. Using the same PN code as the transmitter, the receiver can correlate and collapse the received, spread signal back to the data signal, while other receivers that use other codes or other transmission techniques cannot. Advantages of using spread spectrum communication techniques include robustness with respect to interference caused by other signal sources, the possibility of sharing the same frequency band for different co-existing applications, a potential for high effective data rates and a guarantee on message privacy. These advantages make spread spectrum techniques a preferred technology in order to realize communication systems for use on noisy network media where robustness of the link is crucial, like in wireless local area networks or in industrial environments with high and unpredictable interference levels or in home or indoor office environments.
Systems employing spread spectrum communication methods are well-known.
U.S. Pat. No. 5,359,625 discloses an apparatus and method for transmitting and receiving data on a communication channel making use of spread spectrum communication techniques using direct sequences that approximate a swept frequency waveform.
U.S. Pat. No. 5,022,047 discloses an apparatus for decoding received spread spectrum signals modulated with a PN code.
U.S. Pat. No. 5,309,474 discloses a direct sequence spread spectrum modulator with a signal orthogonalizer. Exemplary embodiments are shown wherein signals are communicated between a cell-site and mobile units using direct sequence spread spectrum communication signals.
U.S. Pat. No. 5,357,541 discloses a transceiver for transmitting and receiving digital data using direct sequence spread spectrum communication techniques. The transceiver also includes circuit means for selecting different PN code bit sequences to provide multiple communication channels on a single transmission medium. Additional means for providing selectable carrier frequencies are also disclosed. The means provide a limited programmability of the transceiver.
U.S. Pat. No. 5,235,615 discloses a method for establishing and communicating synchronous, code division multiple access communications between a base station and a plurality of remote units.
U.S. Pat. No. 5,375,140 discloses a wireless direct sequence spread spectrum digital cellular telephone system. Orthogonal CDMA techniques are used.
U.S. Pat. No. 5,363,401 describes a mechanism for extracting hybrid frequency-hopping/direct sequence signals within a multi-signal type environment.
U.S. Pat. No. 5,414,728 discloses a method and apparatus for bifurcating signal transmission over an in-phase and quadrature-phase spread spectrum communication channel using orthogonal codes on the in-phase and quadrature-phase branch. Both transmitter and receiver are described.
In the article "Silicon Synthesis of a Flexible CDMA/QPSK Mobile Communication Modem," DSP Applications, February 1994, by L. Philips et al., parts of the architecture of a flexible modem chip ("chips" herein to be understood as an integrated circuit) that can be used in a broad class of satellite transceivers is disclosed. This modem chip is called the Programmable Mobile Communication Modem (PMCM) and is programmable to a large extent. The chip is realized as an integrated circuit with a hardware core that is reusable, and therefore adapted for programming for different applications. The PMCM chip is used as one part of a chip set, the other part being any commercially available DSP chip.
A modem is a device that is designed to optimally transmit data over a particular, specified channel. There exists many types of channels. Hence, there exist many types of modems.
This is also true for the particular case of direct sequence spread spectrum (DSSS) modems. From the characteristics of the channel, and the required performance of the modem for that particular channel, the structure of the modem and a large number of modem parameters are selected. There is no systematic way to make these selections in the prior art. It is rather the art of the modem designer that makes a good modem design.
There are a number of methods and tools for supporting the design of large and complex electronic systems such as digital multi-processor systems. These systems comprise many parts which are very different, such as control dominated parts, reactive parts, data flow parts, and structurally described parts. Such systems are referred to as heterogeneous systems. The methods and tools propose a unifying specification paradigm to capture a behavioral specification of a large and complex, heterogeneous digital or mixed digital/analog electronic systems. These methods and tools propose a simulator concept to cope with all aspects of the various parts of the specified system. The methods and tools propose a single implementation target for heterogeneous systems. Examples are named Ptolemy, by the University of Berkeley, Calif., COSSAP by the RWTH Aachen, Germany (now commercialized by CADDIS, a subsidiary from Synopsys) or GRAPE by the University of Leuven, Belgium.